1. Field of the Invention
The present invention relates to a super-resolution material for an optical information recording medium, particularly to a super-resolution material of which a partial region under a focused laser beam changes to melt according to the temperature profile due to light absorption, so as to have different optical transmissivity and reflectivity from its adjacent region.
2. Description of the Background Art
Optical disks include a ROM (Read Only Memory) disk such as a CD-ROM or a DVD-ROM that is able to produce readout signals from pre-pits engraved on a substrate, a WORM (Write Once Read Many) disk that can be only read out once it is written, and a rewritable disk such as a magneto-optic disk that makes use of the Kerr rotation of a linear polarized beam varying with a magnetization direction of a recording film or a phase change optical disk that uses a reflectivity difference between a crystalline space and an amorphous mark on a recording film.
In the optical disk, the size of a focused laser beam to record or reproduce information is determined by the wavelength (xcex) of a laser beam and numerical aperture (NA) of an objective lens, which is given by xcex/NA at a diffraction limit.
For the focused beam at a diffraction limit, a minimum period of the recorded pattern that can be theoretically read out is given by xcex/2NA. Accordingly, in order to accomplish a high density in an optical disk, the wavelength of a laser beam should be reduced, or the NA should be increased to reduce the size of the focused beam.
With the wavelengths of currently available semiconductor lasers limited to a visible ray regime, reduction of a focused beam size may rather be achieved by increase in NA. With an NA significantly greater than those currently used, a tilt margin and a spherical aberration margin of the disk become so small, causing a technical difficulty in designing an optical pick-up head. Furthermore, the conventional method of laser beam through 1.2 or 0.6 mm substrate needs be discarded, leading to the problem of lack of compatibility with the existing disks.
With respect to a ROM type optical disk as well as a rewritable phase change optical disk, another technical approach, so called super-resolution, may be employed to accomplish a high density. In this method, optical and thermal properties of a thin film layer of an optical disk are controlled to partially block the incident focused laser beam, thereby to give rise to an effect of reducing the beam size.
FIG. 1A schematically illustrates the concept of super-resolution for an optical disk, where a super-resolution layer 1 is irradiated with the beam spot 3 focused by the objective lens. FIGS. 1B and 1C show a Gaussian intensity profile 4 of a laser beam and the resulting temperature profile 5.
With reference to FIG. 1A, a portion 1a of the super-resolution layer 1 indicated by slanted lines has different optical properties from those of the rest due to the temperature profile resulting from laser beam irradiation. Since light transmission through the portion 1a is different from that of the adjacent region 1b, the size of the laser beam spot is reduced in effect to that of the portion with slanted lines.
For a successful application to an optical disk, the super-resolution material should satisfy several basic requirements as follows.
Firstly, the transmissivity and the reflectivity difference should be large between the region of changed optical properties and that of no change in optical properties under laser beam irradiation.
Secondly, the super-resolution material should have a high sensitivity to laser power so that optical properties can change as desired with a low laser power.
Thirdly, the material should not be deteriorated during repeated reproduction and/or recording of information.
Fourthly, the super-resolution material should have a fast response to laser beam irradiation in terms of change in optical properties.
Lastly, the material should restore promptly its initial state as laser beam moves away from the area under irradiation.
Conventionally, chalcogenide compounds such as GeSbTe and GeTe or pure metals such as In and Te have been used as super-resolution materials. In these materials, irradiation by a focused laser beam generates a temperature profile such that an area under the beam spot either melts or changes in crystal structure with concomitant changes in optical properties such as transmissivity and reflectivity.
However, a super-resolution material based on a crystal-to-melt transition has shortcomings such as material flow and compositional segregation during repeated readout processes of melting and recrystallization. Therefore, the super-resolution material is progressively degraded with repeated readout in terms of its performance, failing to serve the purpose any longer.
In an effort to overcome the deficiency, there has been proposed a method in which a small amount of a refractory metal or an alloy is added to the fusible matrix in order to suppress material flow by way of enhanced viscosity. Nevertheless, this method does not provide an ultimate solution.
With regard to a super-resolution material utilizing a crystal-to-crystal transition, on the other hand, it is not usually suitable for an optical information recording medium because of inherently slow kinetics of the transition.
An objective of the present invention is to provide a super-resolution material for an optical information recording medium which is capable of rendering a large difference in optical properties between a solid state and a melt state, suppressing material flow to reduce material degradation with repeated use, and generating a super-resolution function with a low laser power.
To realize these and other advantages and to achieve the purpose of the present invention as embodied and broadly described herein, there is provided a composite material thin film in which metal particles with low melting points such as Sn, In, Pb, Bi, Te, Zn Cd, Se, Tl and Po are embedded as dispersions in a dielectric medium such as SiO2, TiO2, ZrO2, HfO2, Al2O3, ZnO, Y2O3, BeO, MgO, WO3, V2O3, SiN, AlN, ZnS, CdS SiC, MgF, CaF2, NaF, BaF2, PbF2, LiF, LaF3 or GaP that is transparent at a laser wavelength in use.
A high density optical information recording medium can be realized by use of a super-resolution layer that is able, in effect, to reduce the focused beam size owing to the characteristic that optical properties such as transmissivity and reflectivity of the composite material thin film change as the metal particles melt.